The present invention relates to a structure for preventing loosening of bolts or nuts after tightening in a screw-fastening tool for tightening a plurality of members together by using bolts or a set of bolts and nuts.
Bolts prevented from loosening through application of adhesive on screw portions of bolts or nuts prevented from loosening such as Hard Locks (trade name) or U-nuts (trade name) are conventionally known to be commercially available. However, such products are arranged in that bolts or nuts are particularly treated or processed and are not adaptable for bolts or nuts of standard type.
Structures have further been proposed for preventing loosening of nuts in which slip surfaces are formed on bearing surfaces of nuts or surfaces of washers such that oscillations of members to be tightened or movements of the members to be tightened caused through thermal expansion/shrinkage are prevented from being transmitted to the nuts. However, since large pressure is applied on bearing surfaces of nuts, costs involved for forming slip surfaces capable of standing such pressure would lead to higher manufacturing costs.
In another known structure, square portions are integrally formed on trunks of bolts while piercing holes of members to be tightened are formed to be of noncircular shape that fit the square portions of the trunks. Since the fit between the square portions of trunks and the noncircular piercing holes will restrain rotation of bolts, it will be unnecessary to fix head portions of the bolts by using wrenches or similar in case the nuts are to be tightened. Thus, tightening operations can be easily performed when employing this structure. On the other hand, such a structure is not applicable to screw-fastening tools in which bolts need to be rotated for the tightening as in screw-fastening tools in which, for instance, a bolt is pierced through a piercing hole of one member to be tightened whereupon this bolt is screwed into a screw hole of another member to be tightened to achieve tightening.
A structure has been suggested in which a protrusion is integrally formed on a trunk of a bolt whereupon this protrusion is inserted into a clearance formed between a circular piercing hole of a member to be tightened and the trunk of the bolt (See Japanese Examined Patent Publication No. 2-114742(1990) and U.S. Pat. No. 5,158,409). The protrusion of the bolt has a height that is larger than xc2xd of a difference between a diameter of the trunk and a diameter of the piercing hole so that the bolt eccentrically rotates when rotating within the piercing hole with the center being somewhat shifted from a central axis of the trunk, thus making a head portion of the bolt slide on a bearing surface while continuously being shifted in a lateral direction. On the other hand, tightening the nut upon screwing the nut into the bolt will result in a larger sliding resistance between the head portion and the bearing surface of the bolt so that the bolt is not allowed to further move upon shifting in lateral directions whereupon the rotation of the bolt will be terminated. Thus, using a bolt formed with such a protrusion will eliminate the necessity of fixing the head portion of the bolt by means of a wrench or the like when tightening the nut to thus enable easy tightening operations. However, in case it is necessary to rotate the bolt around a central axis of the trunk for the tightening as it is the case with a screw-fastening tool in which the bolt is pierced through a circular piercing hole of one member to be tightened whereupon this bolt is screwed into a screw hole of another member to be tightened to achieve tightening, since the one member to be tightened will be shifted and moved through the protrusion of the bolt each time the bolt is rotated so that it is impossible to perform tightening operations.
It is an object of the present invention to provide a structure for preventing loosening that is also applicable to bolts or nuts of standard type.
It is another object of the present invention to provide a structure for preventing loosening that can be obtained at relatively low costs.
It is still another object of the present invention to provide a structure for preventing loosening that is applicable to screw-fastening tools of various structures.
It is still another object of the present invention to provide a structure for preventing loosening with which it is possible to achieve effects of preventing loosening without requiring any special processes or operations after tightening.
For achieving the above object, the present invention provides a structure for preventing loosening of a screw-fastening tool comprising a first member including a piercing hole, a second member including a screw hole, and a bolt including a head portion and a trunk, wherein the first member and the second member are tightened together by inserting the trunk of the bolt through the piercing hole of the first member, screwing the same into the screw hole of the second member and tightening the bolt, wherein a washer is disposed between the head portion of the bolt and the first member, the washer including a projecting portion that is inserted into a clearance formed between the trunk of the bolt and the piercing hole of the first member, wherein the bolt and the washer are allowed to relatively rotate during tightening of the bolt while the bolt and the washer are prevented from relatively rotating in a direction of loosening the bolt after tightening of the bolt.
The present invention further provides a structure for preventing loosening of a screw-fastening tool comprising a first member including a piercing hole, a second member including a screw hole, and a bolt including a head portion and a trunk, wherein the first member and the second member are tightened together by inserting the trunk of the bolt through the piercing hole of the first member, screwing the same into the screw hole of the second member and tightening the bolt, wherein a first and a second washer are disposed between the head portion of the bolt and the first member, the first washer including a projecting portion that is inserted into a clearance formed between the trunk of the bolt and the piercing hole of the first member, and the second washer including a fitting portion that fits with the head portion of the bolt, wherein the first and the second washer are allowed to relatively rotate during tightening of the bolt while the first and the second washer are prevented from relatively rotating in a direction of loosening the bolt after tightening of the bolt.
The above structure for preventing loosening is also applicable to a nut in a screw-fastening tool comprising a plurality of members including piercing holes, a bolt including a head portion and a trunk, and a nut, wherein the trunk of the bolt is passed through the piercing holes of the plurality of members prior to screwing the same to the nut for tightening the nut in order to tighten the plurality of members together.
Since the present invention is arranged to prevent loosening by means of washers disposed on bearing surfaces of bolts or nuts, the present invention is also applicable to bolts or nuts of standard type such that manufacturing costs may be kept relatively low. It is also applicable to screw-fastening tools employing bolts or to screw-fastening tools employing sets of bolts and nuts. It is possible to exhibit loosening preventing effects without the necessity of performing any particular processing or operation after tightening. Economical advantages may be achieved since the washers may be used instead of ordinary washers.